Road surfaces (roads) on which vehicles drive are marked or installed with various road markings such as white lines, yellow lines and cat's eyes constituting traffic lane marking lines. Conventionally the recognition of white lines and other such traffic lane marking lines has, as taught by Patent Reference 1, been done by subjecting an image photographed or taken by image photographing means such as a CCD camera and an image processing ECU to differentiation and binarization processing to detect edges in the image and subjecting point sequences of the detected edge (white line candidate point sequences) to Hough transformation to extract approximated linear components.
Further, when, for example, a white line marked on the road surface is itself physically worn through or partially chipped away, or the white line is indistinct in the photographed image owing to low contrast such as may occur during night driving, the white line cannot be accurately recognized, and, as set out in Patent Reference 2, a technique has been proposed for, in such a case, forming a window in the photographed image, judging the degree of white line wear-off from the density of the white line candidate points in the window, and when wear-off is found, superimposing/compositing a white line candidate point sequences extracted from a photographed image taken a given time earlier on/with the white line candidate point sequences extracted from the current white line image, determining a straight line approximating the composited white line candidate point sequences, and recognizing it as the white line.    Patent Reference 1: Japanese Patent Publication No. Hei 6(1994)-42261    Patent Reference 2: Japanese Laid-Open Patent Application No. 2001-236506
In the technique set out in Patent Reference 2, since the past image to be composited with the current image is an image photographed at a time point earlier by a constant time T0, the recognition accuracy of white lines is not necessarily satisfactory. That is, since the distance of vehicle travel varies with vehicle speed and hence the photographing range of camera moves in the vehicle traveling direction, the traffic lane marking line in the image moves in response to the vehicle travel. Specifically, the higher the vehicle speed is, the more greatly the position of the traffic lane marking line moves. Recognition of the traffic lane marking line by image recognition naturally amounts to recognition of the course of travel (the subject vehicle's or own lane) and since the accuracy with which the direction of the traffic lane marking line is recognized improves with increasing length of the white line components of the traffic lane marking line, it is preferable to composite such that the traffic lane marking line is elongated in appearance, irrespective of presence/absence of the white line component wear and chipping.
Therefore, an object of this invention is to overcome the aforesaid drawbacks and provide a traffic lane marking line recognition system for vehicles which selects an image to be composited in accordance with the vehicle speed, thereby enabling accurate and unerring recognition of the traffic lane marking line.